This invention relates in general to vehicle steering systems and in particular to integration of a fail safe electrically assisted steering system with an autonomous driving system.
Referring now to FIG. 1, there is shown a typical electrically assisted steering system 8 as described in U.S. Pat. No. 6,394,218. The steering system 8 contains a steering spindle 10 having a longitudinal axis L and that is capable of performing a translatory movement in a casing 12. Connected to the steering spindle 10 is a steering linkage (not shown) leading to the steerable wheels of a vehicle.
The translatory movement of the steering spindle 10 in the casing 12 is achieved by means of a recirculating ball nut 14 which may be driven by two positioners 16 disposed in sequence in the interior of the casing 12. Each positioner 16 comprises a servomotor 18 including a rotor 20. The rotors 20 of the servomotors 18 are arranged on a common drive spindle 22 which is non-rotatably connected to the recirculating ball nut 14. The drive spindle 22 together with the rotors 20 is arranged concentrically to the steering spindle 10.
Each positioner 16 also includes a position sensor 24 capable of sensing the rotational position of the corresponding rotor 20. The servomotor 18 and the sensor 24 of each positioner 16 are encapsulated so that a defect in any one servomotor 18 cannot be detrimental to the functioning of the other. Additionally, each positioner 18 is electrically connected to an Electronic Control Unit (ECU) 26 serving to provide control signals to the corresponding servomotor 18. For this purpose each of the ECU's 26 is provided with a supply voltage V so that each servomotor 18 can be supplied with the necessary current.
Also housed in the casing 12 is a position sensor 27 capable of sensing a specific position of the steering spindle 10, for example, its center position. The ECU 26 is able to determine the actual position of the steering spindle in each case on the basis of the signal furnished by the position sensor 27 in conjunction with the signal furnished by the corresponding position sensor 24.
Control of each servomotor 18 occurs in accordance with information made available to the associated ECU 26. For example, each ECU 26 receives a signal from the position sensor 24 indicating the rotational position of the servomotor rotor 20. The ECU 26 further receives a signal from a control unit 28 indicating the setpoint position of the steering spindle 10. This setpoint value is established by the control unit as a function of external parameters, for example, depending on the setpoint value of a steering angle made available by a steering angle sensor for the vehicle steering wheel. The control unit 28 receives, in turn, signals representing the force acting in the steering spindle 10 and thus permits conclusions as to the cornering force acting on the steerable vehicle wheels.
Unlike error tolerant systems in which a mechanical connection is typically provided between the steering wheel and the steering spindle, forming a fall-back should any component of the steering system develop a fault, the steering system 8 is configured to include redundancy. Should one of the servomotors become defective the other servomotor is still able to ensure continuing steerability of the vehicle as long as it is on the move. Should one of the position sensors 24 become defective the remaining sensor is still able to furnish the required information. Likewise, should one of the ECU's 26 fail, the remaining ECU will continue to provide control signals to its corresponding servomotor 18.
New systems for directional control of vehicles are being continuously developed. These systems may be autonomous in that they may implement control actions in response to sensor inputs and independently of the vehicle operator. Some autonomous systems may implement vehicle directional control through the vehicle steering system and/or brake system. Accordingly, when a vehicle includes electrically assisted steering, it would be desirable to integrate autonomous directional control systems that involve the vehicle steering system with the steering system in order to reduce the over-all complexity of the vehicle. It also would be desirable to reduce the complexity and number of components required for the electrically assisted steering system.